1. Field of the Invention
This invention relates to an assembly of components for a molten carbonate fuel cell comprising a laminate of at least one electrode and an electrolyte matrix and a process for making the assembly.
2. Description of the Prior Art
Known molten carbonate fuel cells consist of a cathode and an anode between which is disposed an electrolyte matrix tape formed from a slurry of lithium aluminate in an organic vehicle containing a plastic binder and additives that impart good slurry properties. The slurry is cast onto a flat Teflon.RTM. substrate by a doctor blade and the organic solvent is allowed to evaporate. The dried tape is flexible as a result of the remaining plastic binder which is removed by thermal degradation after assembly of the fuel cell. Single or multi-piece matrix tapes are assembled in the fuel cell with a cathode on one side and an anode on the other side. The matrix is impregnated with molten carbonate, contained in an electrolyte tape or in the pores of the porous electrode, by capillary action during heat up of the fuel cell. As the fuel cell is heated, the plastic binder in the matrix is removed and the carbonate in the electrolyte tape or the electrode becomes molten and is wicked into the matrix The fuel cells are typically combined into a fuel cell stack and have positioned between them a separator plate which separates the anode of one fuel cell from the cathode of the adjacent fuel cell.
More specifically, a molten carbonate fuel cell consists of an electrolyte structure containing lithium aluminate and a mixture of alkali carbonates. A cathode and an anode are attached to the two faces of the matrix, one on each side thereof. Each electrode is contained within a metal plate compartment, the plate resembling a framed structure with a peripheral frame acting as a "wet seal". The electrodes fit within the "active area" which is the area inside the frame and the electrode height is flush with the "wet seal" frame. The matrix extends all the way to the outer edge of the "wet seal". At the fuel cell operating temperature of about 650.degree. C., the molten electrolyte forms a seal against the "wet seal" frame to prevent gas leakage from the electrode compartments. Normally, the internal edges of the anode and cathode wet seal frames are aligned with each other.
U.S. Pat. No. 4,538,348 teaches casting electrolyte matrix tapes from a mixture of inert particles smaller than one micron, such as lithium aluminate, corrosion resistant ceramic particulates for crack resistance and a temporary plastic binder, all heated "in cell" to remove the binder. Upon cell assembly, active electrolyte is furnished by a prefilled anode.
There are several disadvantages associated with conventional fuel cell assembly and start-up. "In-cell" binder removal and electrolyte impregnation of the matrix requires a prolonged and carefully controlled process. In addition, the completed fuel cell requires the formation of a "wet seal" frame by the molten electrolyte with the peripheral frame of the metal plate compartment containing each electrode to prevent gas leakage into the electrode compartments. Finally, the clamping force applied to the cell easily causes cracking of the matrix resulting in gas crossover between electrodes.
Densification of at least the edge portions of fuel cell components is known for improving the gas edge seal for gas porous fuel cell components. U.S. Pat. No. 4,269,642 and related U.S. Pat. No. 4,365,008 teach an improved gas edge seal for gas porous fuel cell components and a process for fabricating such fuel cell components in which a fully graphitized gas porous, resin bonded carbon fiber sheath having edge portions which are more dense than a central portion is formed by forming an intermediate product comprising carbon fibers and a thermosetting resin which is not fully cured, the intermediate product having increased thickness along its edges, simultaneously densifying at least the edge portions by compressing them to a thickness which is substantially the same as the desired final thickness of the central portion of the components, curing the part and further heating the part to carbonize the resin and subsequently graphitize the resin and carbon fibers.
U.S. Pat. No. 4,652,502 and related U.S. Pat. No. 4,756,981 disclose a process for making porous plates for an electrochemical cell in which the peripheral edge regions of the plates are provided with smaller pores than the remainder of the plate and edge sealing is effected by a suspension of sealing material forced into the edge region producing an edge region having a higher density than the remaining plate. Similarly, U.S. Pat. No. 4,786,568 teaches an electrode plate assembly for use in a fuel cell power plant in which the edges of a porous substrate plate are densified by impregnating a suspension of finely divided material in the form of an "ink" into the interstitial spaces in the plate edges to form an integral filler band around a catalyst layer which has been previously applied to one surface of the substrate plate. A different approach is taught by U.S. Pat. No. 4,659,635 in which an improved porous matrix for containing molten carbonate electrolyte in a fuel cell stack consists of a substantially flat sheet of porous ceramic material having a generally smaller pore size average in the central body of the sheet and a generally larger second pore size average in the edge portion of the sheet, the larger pore sizes upon wetting with molten carbonate leaving void a major portion of the internal porosity.